Protein-stabilized oil-in-water emulsions are widely utilized in the food, cosmetics and pharmaceutical industries. These emulsions consist of protein-coated lipid droplets dispersed in an aqueous continuum. Conventionally, these emulsions are created by homogenizing an oil phase with an aqueous phase containing surface-active proteins, such as to casein, soy proteins, egg proteins or whey proteins. The protein molecules adsorb to the surface of the droplets produced during homogenization where they form a protective coating that prevents them from aggregating, e.g., flocculating and/or coalescing. In addition, the adsorbed proteins reduce the oil-water interfacial tension, thereby facilitating the further disruption of lipid droplets during homogenization and leading to smaller droplet sizes. At present, proteins can only be used successfully as emulsifiers in a limited range of materials because of their high sensitivity to changes in solution pH and ionic strength. Protein-coated lipid droplets are primarily stabilized by electrostatic repulsion, consequently they tend to aggregate when the pH moves close to the isoelectric point of the proteins (due to reduction of the ζ-potential) or when the ionic strength increases above a certain level (due to increased electrostatic screening). In addition, protein-coated lipid droplets are often susceptible to aggregation when they are heated above the thermal denaturation temperature of the adsorbed proteins because this increases the hydrophobic attraction between them.
Recently, an interfacial engineering technology, based on the layer-by-layer (LbL) electrostatic deposition technique, has been used to improve the stability of protein-coated lipid droplets to environmental stresses, such as pH, ionic strength and temperature. Initially, a “primary emulsion” consisting of lipid droplets coated by a layer of charged globular proteins is produced using conventional homogenization. Then, a “secondary emulsion” is formed by depositing an oppositely charged polyelectrolyte (e.g., an ionic polysaccharide) onto the surface of the protein-coated lipid droplets. This procedure can be repeated a number of times by successively depositing layers of oppositely charged polyelectrolytes onto the surfaces of the lipid droplets so that multilayered interfacial coatings are formed. Rational selection of polyelectrolyte characteristics and deposition conditions enables one to carefully control interfacial characteristics, such as thickness, charge, permeability, environmental responsiveness and functionality.
One potential limitation of the electrostatic deposition method for certain applications is that interfacial protein-polysaccharide complexes are only held together by electrostatic attraction. Consequently, the polysaccharide layer may dissociate from the protein-coated lipid droplet surfaces when the pH is varied so that the protein and polysaccharide have opposite charges, or when the ionic strength is increased above a certain level. As a result, there remains an on-going search in the art to provide emulsion systems that remain intact with changes in pH or ionic strength of the surrounding solution.